1. Field of the Invention
The invention relates to a stator of a torque sensor, which is formed of a laminated structure of electromagnetic steel sheets.
2. Discussion of Background
A twin resolver-type torque sensor as described in Japanese Patent Application Publication No. 2008-58026 (JP 2008-58026 A) has been conventionally used as one type of torque sensor. The torque sensor includes a first resolver that detects a rotation angle of a first rotary shaft and a second resolver that detects a rotation angle of a second rotary shaft. The first rotary shaft and the second rotary shaft are coupled to each other via a torsion bar. The torque sensor described in JP 2008-58026 A calculates the rotation angle of the first rotary shaft and the rotation angle of the second rotary shaft on the basis of corresponding signals output from the two respective resolvers, and obtains the amount of torsion of the torsion bar by computing the difference between the calculated rotation angles. A torque that acts on the rotary shaft is calculated on the basis of the amount of torsion.
A stator of each resolver is fixed to, for example, a case that accommodates the resolvers. As such a fixation structure, there is a structure described in Japanese Patent Application Publication No. 2005-315645 (JP 2005-315645 A). In each of the resolvers described in JP 2008-58026 A, a flange that has insertion holes is formed on the outer periphery of each stator core. In addition, the distal end portion of each bolt protrudes from the inner wall face of the case. En the resolver described in JP 2005-315645 A, after the distal end portion of each bolt that protrudes from the inner wall face of the case is passed through a corresponding one of insertion holes of a stator core, a nut is screwed to the distal end portion of each bolt. Then, the stator core is fixed to the case by fastening the nuts.
In the case of such a fixation structure, when each nut is fastened, stress is generated in the stator core due to fastening axial force. Such stress exerts influences on the magnetic characteristic of the stator core, thereby leading to, for example, an increase in a core loss of the stator core and a decrease in magnetic flux density. This causes a decrease in the ratio of the output voltage of a detection coil to the input voltage of an exciting coil (voltage transformation ratio), and a decrease in the accuracy of detection of a rotation angle.